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United States Patent |
5,691,498
|
Fogle, Jr.
|
November 25, 1997
|
Hermetically-sealed electrically-absorptive low-pass radio frequency
filters and electromagnetically lossy ceramic materials for said filters
Abstract
An electromagnetically lossy liquid- or gas-tight fusion seal for use as a
low pass radio frequency signal filter constructed as a matrix of glass
binder and ferrimagnetic and/or ferroelectric filler. Metal cased
electrical filters are made by reflowing the material to form fused
glass-to-metal seals and incorporating electrical thru-conductors therein
which may be formed as inductive windings.
Inventors:
|
Fogle, Jr.; Homer William (Mesa, AZ)
|
Assignee:
|
TRW Inc. (Cleveland, OH)
|
Appl. No.:
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227677 |
Filed:
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April 14, 1994 |
Current U.S. Class: |
102/202.2; 313/134 |
Intern'l Class: |
F42B 003/188 |
Field of Search: |
313/134
102/202.1,202.2,202.3,202.4
|
References Cited
U.S. Patent Documents
2292216 | Aug., 1942 | Doran | 313/11.
|
2311647 | Feb., 1943 | Doran | 313/142.
|
2821139 | Jan., 1958 | Apstein et al. | 102/202.
|
3002458 | Oct., 1961 | Haas | 102/202.
|
3208379 | Sep., 1965 | McKee et al. | 102/202.
|
3213791 | Oct., 1965 | Schnetter | 102/202.
|
3227083 | Jan., 1966 | Moses et al. | 102/202.
|
3380004 | Apr., 1968 | Hansen | 333/184.
|
3720862 | Mar., 1973 | Mason | 106/39.
|
4048714 | Sep., 1977 | Huutt | 29/603.
|
4232277 | Nov., 1980 | Dickens et al. | 330/4.
|
4741849 | May., 1988 | Naito et al. | 501/32.
|
4848233 | Jul., 1989 | Dow et al. | 102/202.
|
4855261 | Aug., 1989 | Mizuno et al. | 501/76.
|
5036768 | Aug., 1991 | Dow et al. | 102/202.
|
5036769 | Aug., 1991 | Dow et al. | 102/202.
|
5120366 | Jun., 1992 | Havada et al. | 106/486.
|
Other References
"Chemical Bonding At Glass-to-Metal Interfaces", Pask, J.A., MD-vol. 4, pp.
1-7, presented at The Winter Annual Meeting of The American Society of
Mechanical Engineers, Boston, MA Dec. 13-18, 1987.
|
Primary Examiner: Johnson; Stephen M.
Attorney, Agent or Firm: Earley; John F. A., Earley, III; John F. A.
Parent Case Text
This patent application is a continuation-in-part patent application of
U.S. patent application Ser. No. 07/832,473, filed Feb. 7, 1992, now U.S.
Pat. No. 5,367,956, which is incorporated herein by reference.
Claims
I claim:
1. A monolithic combination electrical low-pass radio frequency absorbent
filter and mechanical gas-tight seal apparatus comprising
an electrically conductive metallic casing having a passageway therethrough
and an interior wall,
at least one metallic electrode extending through said passageway and not
contacting said casing, and
means for attenuating high frequency electrical signals and for blocking
the passage of gas through the passageway, said means including
a solid electromagnetically lossy substantially gas-impermeable plug fused
to the interior wall of said casing passageway and to said electrode so as
to partially imbed said electrode within said plug and completely span the
remaining free cross section of said passageway,
the plug comprising a dense vitreous ceramic matrix of (a) a
multi-component glass binder, 5-50% by weight, and (b) at least one
electromagnetically lossy ferrimagnetic and/or ferroelectric filler
interspersed throughout, 50-95% by weight, said ceramic matrix having
mechanical and electrical properties of
linear expansion coefficient in the range of 3 to 20 ppm/.degree.C.,
helium permeability not greater than 2.times.10.sup.-11 darcys,
working point in the range of 400.degree. to 1000.degree. C.,
strain point in the range of 250.degree. to 700.degree. C.,
Curie temperature in the range of 130.degree. to 600.degree. C.,
DC electrical volume resistivity in excess of 100 ohm-cm,
dielectric strength in excess of 150 volts/mil, and
unguided wave attenuation constant greater than 1 neper/meter at 1 MHz, and
greater than 5 nepers/meter at 10 MHz and above.
2. The apparatus of claim 1, the binder including a Lead Borosilicate glass
composed of Lead Oxide, Lead Silicate, Boron Oxide and Aluminum Oxide.
3. The ceramic material of claim 1, the glass binder including a Lead
Boroaluminosilicate glass composed of Silica, Aluminum Oxide, Boron Oxide,
and Lead Oxide.
4. The ceramic material of claim 1, the lossy ferrimagnetic filler
comprising spinel ferrite having the general formula (AaO).sub.1-x
(BbO).sub.x Fe.sub.2 O.sub.3, where Aa and Bb are divalent metal cations
selected from the group consisting of Ba, Cd, Co, Cu, Fe, Mg, Mn, Ni, Sr,
and Zn, and x is a fractional number on the interval ›0,1).
5. The ceramic material of claim 1, the lossy ferroelectric filler selected
from the group consisting of perovskite titanate of the type
(CcO)TiO.sub.2, and a zirconate of the type (CcO)ZrO.sub.2, where Cc is a
divalent metal cation of Ba, La, Sr or Pb.
6. The ceramic material of claim 1, the lossy ferroelectric filler
comprising a perovskite La-modified Lead Zirconium Titanate.
7. A monolithic combination electrical low-pass radio frequency absorbent
filter and mechanical gas-tight seal apparatus, said combination
comprising
an electrically conductive metallic casing having a passageway therethrough
and an interior wall,
at least one metallic electrode extending through said passageway and not
contacting said casing, and
means for attenuating high frequency electrical signals and for blocking
the passage of gas through the passageway, said means including
a solid electromagnetically lossy substantially gas-impermeable plug fused
to the interior wall of said casing passageway and to said electrode so as
to partially imbed said electrode within said plug and completely span the
remaining free cross section of said passageway,
wherein the imbedded electrode is formed in the shape of a curvilinear
winding or in the shape of a curvilinear winding with reversals in
direction,
the plug comprising a dense vitreous ceramic matrix of (a) a
multi-component glass binder, 5-50% by weight, and (b) at least one
electromagnetically lossy ferromagnetic and/or ferroelectric filler
interspersed throughout, 50-95% by weight, said ceramic matrix having
mechanical and electrical properties of linear expansion coefficient
values in the range of 3 to 20 ppm/.degree.C., helium permeability not
greater than 2.times.10.sup.-12 darcys, working point in the range of
400.degree. to 1000.degree. C., strain point in the range of 250.degree.
to 700.degree. C., Curie temperature in the range of 130.degree. to
600.degree. C., DC electrical volume resistivity in excess of 100 ohm-cm,
dielectric strength in excess of 150 volts/mil, and unguided wave
attenuation constant greater than 1 neper/meter at 1 MHz, and greater than
5 nepers/meter at 10 MHz and above,
the binder including a Lead Borosilicate glass composed of Lead Oxide, Lead
Silicate, Boric Oxide and Aluminum Oxide, or a Lead Boroaluminosilicate
glass composed of Silica, Aluminum Oxide, Boron Oxide, and Lead Oxide,
the lossy ferrimagnetic filler comprising spinel ferrite having the general
formula (AaO).sub.1-x (BbO).sub.x Fe.sub.2 O.sub.3, where Aa and Bb are
divalent metal cations selected from the group consisting of Ba, Cd, co,
Cu, Fe, Mg, Mn, Ni, Sr and Zn, and x is a fractional number on the
interval 0,1),
the lossy ferroelectric filler selected from the group consisting of
perovskite titanate of the type (CcO)TiO.sub.2, and a zirconate of the
type (CcO)ZrO.sub.2, where Cc is a divalent metal cation selected from the
group consisting of Ba, La, Sr or Pb, and a perovskite La-modified Lead
Zirconium Titantate.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to dissipative hermetically sealed electrical filter
assemblies which incorporate electromagnetically lossy ceramic materials
to provide a low-pass frequency response.
2. Description of the Prior Art
Radio frequency interference (RFI) suppression filters having a low-pass
characteristic are commonly incorporated in electrical interconnection
devices or in electrical devices as integral subassemblies to insure that
unwanted radio frequency signals are suppressed while allowing the passage
of direct current (DC) and low frequency alternating current (AC) signals.
This RFI suppression function is sometimes required to insure the
unimpeded operation of RF sensitive electronic equipment in an intensive
RF signal environment or, alternatively, to prevent the conductive or
radiative emission of RF energy from electronic devices. The RFI
suppression function is of considerable concern in the design of
electroexplosive devices (EEDs) where the failure to suppress RF energy
might lead directly to the unpropitious functioning of an explosive or
propellant charge. Such filters must pass direct currents with negligible
internal loss.
In many cases, electrical devices incorporating these RFI filters are also
required to provide a gas-tight seal to protect sensitive components or
materials contained within an enclosure. Heretofore, the electrical
low-pass filters and the mechanical gas- or liquid-tight seals required by
these devices have been separate and distinct components. Many EEDs
incorporate a hermetically sealed chamber for their energetic chemical
material that is vulnerable to degradation by the intrusion of water
vapor. Electrical access to this chamber is obtained by a high integrity
glass-to-metal seal that incorporates imbedded electrical thru-conductors,
hereafter called electrodes. Similarly, many bulkhead mounted connectors
also incorporating RFI suppression filters that are used in aerospace
applications are constructed using glass- or ceramic-to-metal sealing
techniques to achieve required gas- and liquid-tightness.
Absorptive filters are those that dissipate applied RF power within a solid
medium in the form of heat which must be efficiently conducted to the
environment. The loss mechanism may be electrical, magnetic or a
combination thereof. These lumped- or distributed-element
dielectromagnetic structures may be complemented with associated reactive
structures (series inductances and shunt capacitances) to achieve desired
electrical network characteristics.
Electrically dissipative ceramics formed primarily from alumina and silicon
carbide are described in L. E. Gates, Jr., et al. U.S. Pat. No. 3,538,205
issued on Nov. 3, 1970 for "Method of Providing Improved Lossy Dielectric
Structure For Dissipating Electrical Microwave Energy," and in L. E.
Gates, Jr., et al. U.S. Pat. No. 3,671,275 issued on Jun. 20, 1970 for
"Lossy Dielectric Structure For Dissipating Electrical Microwave Energy."
Electrical loss tangents as high as 0.6 are reported. L. E. Gates, Jr., et
al. U.S. Pat. No. 3,765,912 issued on Oct. 16, 1973 for "MgO-SiC Lossy
Dielectric for High Power Electrical Microwave Energy" reports a further
development based on a matrix of magnesia and silicon carbide. However,
these compositions feature negligible magnetic loss, high porosity, high
melting points, and poor wetting characteristics when in the liquid state.
As such, they are unsuitable for forming fusion seals with metallic
members.
Magnetically dissipative materials having acceptably high magnetic loss
tangents and DC volume resistivities are commercially available in the
form of spinel ferrites. E. C. Snelling in Soft Ferrites, Properties and
Applications (Second edition) (Butterworths, Stronham Mass., 1988)
describes the electromagnetic properties of these materials. P. Schiffres
in "A Dissipative Coaxial RFI Filter", IEEE Transactions on
Electromagnetic Compatibility (January 1964, pp. 55-61), describes the
application of these materials for constructing lossy transmission line
filters and J. H. Francis, in "Ferrites as Dissipative RF Attenuators,"
Technical Memorandum W-11/66, U.S. Naval Weapons Laboratory, Dahlgren Va.
(1966), describes their application as EED attenuation elements.
Various glass sealing compositions have been developed for bonding ferrite
shapes to one another as reported in J. F. Ruszczyk U.S. Pat. No.
3,681,044 issued on Aug. 1, 1972 for "Method of Manufacturing Ferrite
Recording Heads With a Multipurpose Devitrifiable Glass," R. Huntt U.S.
Pat. No. 4,048,714 issued on Sep. 20, 1977 for "Glass Bonding or
Manganese-Zinc Ferrite," and Y. Mizuno et al. U.S. Pat. No. 4,855,261
issued on Aug. 8, 1989 for "Sealing Glass." These compositions do not
feature the electromagnetically lossy characteristics that would render
them useful as RF absorbers.
J. A. Pask discusses CHEMICAL BONDING AT GLASS-TO-METAL INTERFACES in an
article published in the TECHNOLOGY OF GLASS, CERAMIC, OR GLASS-CERAMIC TO
METAL SEALING presented at The Winter Annual Meeting of the American
Society of Mechanical Engineers, Boston, Mass., Dec. 13-18, 1987. This
paper discloses that the fusion joint interface between a reflowed
glass-like ceramic and the substrate to which it is bonded, be it a
ferrite or a metal structure, is a chemically distinct region.
Assemblies incorporating magnetically lossy RF absorptive filter elements,
typically spinel ferrites in the form of sintered beads, and physically
distinct mechanical seal elements, typically fused glass-to-metal
structures, are described in T. Warnhall U.S. Pat. No. 3,572,247 issued on
Mar. 23, 1971 for "Protective RF Attenuator Plug for Wire-Bridge
Detonators," J. A. Barret U.S. Pat. No. 4,422,381 issued on Dec. 27, 1983
for "Ignitor With Static Discharge Element and Ferrite Sleeve," and H. W.
Fogle U.S. patent application No. 07-706211 executed on May 28, 1991, for
"Filtered Electrical Connection Assembly Using Potted Ferrite Element."
These designs require separate processing steps to form the filter and
seal elements.
Assemblies incorporating electrically lossy RF absorptive filter elements,
typically ferroelectric materials such as Barium Titanate (BaTiO.sub.3) in
the form of tubular capacitors, and physically distinct mechanical seal
elements are described in W. G. Clark U.S. Pat. No. 3,840,841 issued on
Oct. 8, 1974 for "Electrical Connector Having RF Filter," K. S. Boutros
U.S. Pat. No. 4,187,481 issued on Feb. 5, 1980 for "EMI Filter Connector
Having RF Suppression Characteristics," and S. E. Focht U.S. Pat. No.
4,734,663 issued on Mar. 29, 1988 for "Sealed Filter Members and Process
For Making Same."
Certain automotive spark plugs unify the RF filter and mechanical seal
functions in a glassy ceramic structure that forms a fused seal. For
example, G. L. Stimson U.S. Pat. No. 4,112,330 issued on Sep. 5, 1978 for
"Metallized Glass Seal Resistor Compositions and Resistor Spark Plugs," K.
Nishio et al. U.S. Pat. No. 4,224,554 issued on Sep. 23, 1980 for "Spark
Plug Having a Low Noise Level," M. Sakai U.S. Pat. No. 4,504,411 issued on
Mar. 12, 1985 for "Resistor Composition For Resistor-Incorporated Spark
Plugs," and G. L. Stimson U.S. Pat. No. 4,795,944 issued on Jan. 3, 1989
for "Metallized Glass Seal Resistor Composition," describe ceramic
composition hermetic seals that also act as series connected electrically
dissipative resistances, typically 5000 ohms, to attenuate RF energy
generated at the spark gap so as to reduce RFI emissions from the vehicle
ignition system. These designs depend entirely upon ohmic and dielectric
loss mechanisms to dissipate RF energy. More significantly, they do not
have metallic electrically conducting electrodes that pass through the
glassy seal region with the result that DC losses are significant. These
factors render this technology useless for the manufacture of electrical
thru-bulkhead fittings, connectors and EEDs where DC continuity is an
essential performance requirement.
Plastics with ferrimagnetic or ferroelectric fillers that are intended for
use as RF signal attenuating media are described in H. J. Sterzel U.S.
Pat. No. 4,879,065 issued on Nov. 7, 1989 for "Processes of Making
Plastics Which Absorb Electromagnetic Radiation and Contain Ferroelectric
and/or Piezoelectric Substances." Such plastics allow the design of
attenuating filters that have imbedded electrodes shaped in useful
inductive configurations, e.g. spirals and helical windings. However,
these materials do not have the mechanical durability and chemical
resistance required for mechanical gas- and liquid-tight seals,
particularly at extreme hot and cold temperatures or in corrosive
environments.
Filters featuring spiral shaped electrodes imbedded in lossy ferrimagnetic
ceramics are reported in Dow et. al. U.S. Pat. No. 4,848,233 issued on
Jul. 18, 1989 for "Means For Protecting Electroexplosive Devices Which Are
Subject To A Wide Variety Of Radio Frequency." These fragile high-porosity
devices can not simultaneously serve as fluid sealing elements.
While filter/seal equipped thru-bulkhead fittings, connectors, EEDs and
spark plugs such as those described in the prior art patents have met with
considerable success, they nevertheless suffer from the disadvantage of
complexity in that they require a multiplicity of constituent parts and
various means for joining same together to achieve the electrical,
mechanical and heat transfer functions intended. This complexity leads to
significant manufacturing cost, particularly if the filter designs are not
amenable to assembly by high speed machinery.
SUMMARY OF THE INVENTION
It is an object of this invention to provide combination electrical low
pass RFI suppression filter and gas-tight seal having low cost and robust,
compact and simplified construction.
Another object of this invention is to provide an electromagnetically lossy
glass-like ceramic material suitable for forming low reflow temperature
fusion seals incorporating imbedded thru-conductor electrodes of various
useful shapes, e.g. straight pins, spiral windings with and without
reversals in direction and helical windings with and without reversals in
direction, that act as low-pass electrical networks. These seals feature
improved manufacturability and electrothermal performance over designs now
available.
These and other objects are accomplished by providing a method for
constructing low-pass dissipative RFI suppression filters with intrinsic
hermetic seals. Furthermore, the design for the filters provides
inherently efficient power handling capacity and mechanical ruggedness.
The inventive filter comprises a modified sealing glass, hereafter called
a ceramic material, suitable for manufacturing electrical ceramic-to-metal
seals that are gas-tight and highly lossy with respect to the transmission
of radio frequency signals. The inventive ceramic material is a dense
composite matrix formed from a glass binder and an electromagnetically
lossy filler comprised of a spinel structured ferrimagnetic material
and/or perovskite structured ferroelectric material. The inventive
structure of the filter/seal employs chemically bonded fusion joints to
achieve glass-to-metal adhesion of the ceramic material to adjoining
metallic members.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an end view of one embodiment of a filter-seal assembly of the
invention with two straight thru-conductor electrodes;
FIG. 2 is a vertical cross-sectional view taken approximately on the line
2--2 of FIG. 1;
FIG. 3 is an end view of another embodiment of a filter/seal assembly of
the invention with a single thru-conductor electrode formed in the shape
of a helical winding;
FIG. 4 is a vertical cross-sectional view taken approximately on the line
4.4 of FIG. 3, and
FIG. 5 is a vertical cross-sectional view of a manufacturing process
fixture, and the filter/seal assembly of FIG. 1 situated therein.
FIG. 6 is a vertical cross-sectional view of a filter-seal incorporated as
a subassembly of an electroexplosive device.
FIG. 7 is a vertical cross-sectional view of a filter-seal incorporated as
a subassembly of an automotive spark plug.
FIG. 8 is an alternative embodiment to the vertical cross-section shown in
FIG. 4.
It should of course be understood that the description and drawings herein
are merely illustrative and that various modifications and changes may be
made in the structure disclosed without departing from the spirit of the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now more particularly to the drawings and FIGS. 1 and 2 thereof,
one embodiment of a filter-seal assembly 10 of the invention is disclosed.
The filter-seal assembly 10 includes an electrically conductive metallic
casing 13 having a passageway 17 therethrough. Two metallic electrodes 14
extend through and beyond the passageway 17 of the metallic casing 13. A
solid plug of ceramic material 15 is provided, to be described, and which
is fused, i.e., chemically bonded by a reflow and surface wetting process
at elevated temperature, to the casing 13 and to the electrodes 14 so as
to span the passageway 17, thereby forming a gas-tight electromagnetically
lossy seal. A chemically bonded fusion joint 13a is achieved between
metallic casing 13 and ceramic plug 15, and chemically bonded fusion
joints 15a are achieved between plug 15 and electrodes 14, by liquid-solid
wetting of the ceramic materials melted glass binder to the metal surfaces
and subsequent cooling of said materials.
Referring now more particularly to FIGS. 3 and 4 of the filter/seal
assembly 20 of the invention, another embodiment is disclosed. The
filter/seal assembly 20 includes a metallic casing 23 having a passageway
27 therethrough and electrode 24 extends through/and/beyond the casing 23
which is illustrated as being of helical shape. A solid plug 25 of ceramic
material is provided, to be described, and which is fused to the casing 23
and the electrode 24 so as to span the passageway 27 hereby forming a
gas-tight electromagnetically lossy seal. A chemically bonded fusion joint
23a is achieved between metallic casing 23 and ceramic plug 25, and
chemically bonded fusion joints 25a are achieved between plug 25 and
electrodes 24, by liquid-solid wetting of the ceramic material's melted
glass binder to the metal surfaces and subsequent cooling of said
materials.
FIG. 5 shows non-metallic heat-resistant fixture 31 used to fabricate the
filter-seal depicted in FIGS. 1 and 2. The fixture 31 includes base 35,
pin aligner 37, and cover 33. The casing 13 rests in base 35 with the
lower end of the electrodes being fitted into the pin aligner 37 in base
35. Cover 33 covers the filter-seal assembly and is supported by base 35.
The base 35, cover 33, and pin aligner 37 hold the casing 13 and the
electrodes 14 in fixed relation relative to each other.
Referring now more particularly to FIG. 6, an embodiment of the filter/seal
assembly in the form of an electroexplosive device 40 is depicted. A solid
plug 42 of electromagnetically lossy glass-like ceramic material is
provided which is situated within the passageway 45 of a metallic casing
43 and joined to the inner wall of said casing 43 and also to the
electrode 50 so that a plug-to-casing fusion joint 44 and a
plug-to-electrode fusion joint 46, respectively, are obtained uniformly at
all points of contact between these respective members.
A resistive bridgewire 48 is bonded to the electrode 50 and to the casing
43. A metal charge cup 47 fully loaded with a pyrotechnic composition 41
is joined and sealed to the casing 43 in such a manner as to bring the
pyrotechnic composition 41 into intimate contact with the bridgewire 48.
The electrode 50 emanating from the plug 42 and a casing contact 49 bonded
to the casing 43 provide electrical terminations for the bridgewire
circuit and, as such, comprise the electrical signal input port. The
structure provides a gas-tight hermetically sealed containment for the
pyrotechnic composition 41 by virtue of the gas-impermeable solid plug 42
and the fusion joints 44 and 46. The structure also provides a low pass
distributed element absorptive RFI suppression filter between the input
port and the bridgewire 48 termination.
Referring now more particularly to FIG. 7, an embodiment of the filter/seal
assembly in the form of an automotive spark plug 60 is depicted. A solid
plug 62 of electromagnetically lossy glass-like ceramic material is
provided which is situated within the passageway 70 of a metallic casing
64 and joined to the inner wall of said casing 64 and also to the center
electrode 61 so that a plug-to-casing fusion joint 68 and a
plug-to-electrode fusion joint 67 are obtained uniformly at all points of
contact between these respective members. A ceramic insulator 63 is joined
to the casing to form an electrically insulating extension of said casing
64. A spacing between a ground electrode 65 bonded to the casing 64 and
the center electrode 61 emanating from the plug 62 forms a spark gap 69.
The center electrode 61 emanating from the plug 62 comprises a high
voltage terminal 66 that provides a low-pass electrical access to the
spark gap 69. The structure provides a gas-tight hermetic seal between the
spark gap 69 situated in a closed combustion chamber (not depicted) and
the external environment. The structure furthermore provides attenuation
of spurious RF energy that is generated at the spark gap 69 within said
combustion chamber and would otherwise be conducted back through the
electrical circuitry connected to the high voltage terminal 66.
The ceramic plugs 15, 25, 42 and 62 are of an electromagnetically lossy
glass-like ceramic material. This material comprises a dense matrix which
includes a glass binder and an electromagnetically lossy filler by weight
of 50-95% interspersed throughout the matrix.
The electrode may be linear or curvilinear (e.g.., spiral windings with or
without reversals in direction, and helical windings with or without
reversals in direction). A single electrode or a plurality of electrodes
may be used in each filter/seal assembly 10, 20, 40 and 60.
It should be noted that the plugs 15, 25, 42 and 62 may be pre-formed with
through holes (not shown) prior to insertion in casings 10, 20, 43 and 64
with later placement of the conductors 14, 24, 50 and 61 and reflowed at
elevated temperature for sealing to be described.
Acceptable binders include, but are not limited to, Lead Borosilicate and
Lead Aluminoborosilicate glasses which include oxides of Al, B, Ba, Mg,
Sb, Si and Zn. Commercially available materials in the form of finely
ground frits include CORNING (Corning N.Y.) high temperature ferrite
sealing glasses, e.g. #1415, #8165, #8445, CORNING low temperature ferrite
sealing glasses, e.g. #1416, #1417, #7567, #7570 and #8463, and FERRO
CORPORATION (Cleveland Ohio) low temperature display sealing glasses, e.g.
#EG4000 and #EG4010.
Acceptable ferrimagnetic fillers include, but are not limited to spinel
structured ferrites of the type (AaO).sub.1-x (BbO).sub.x Fe.sub.2 O.sub.3
where Aa and Bb are divalent metal cations of Ba, Cd, Co, Cu, Fe, Mg, Mn,
Ni, Sr or Zn, and x is a fractional number on the semi-open interval
›0,1). Sintered Manganese-Zinc and Nickel-Zinc spinel ferrite powders such
as FAIR-RITE PRODUCTS (Wallkill N.Y.) #73 and #43, respectively, are
examples.
Acceptable ferroelectric fillers include, but are not limited to,
perovskite titanates of the type (XxO)TiO.sub.2 and perovskite zirconates
of the type (XxO)ZrO.sub.2 where Xx denotes divalent metal cations of Ba,
La, Sr or Pb. Barium titanate, (BaO)TiO.sub.2, is a typical species. Other
acceptable fillers include electrically lossy La-modified Pb(Zr,
Ti)O.sub.3 perovskite ceramics known as PLZTs.
The electromagnetically lossy ceramic mixture is formed by mixing the
binder and filler in a ball mill with ceramic media in a volatile organic
carrier liquid with a forming agent and fatty acid dispersant. This
invention includes compositions consisting of 5-50% by weight of binder
and 50-95% by weight of filler. The resulting mixture is then dried.
Filter/seals may be constructed directly from this dried mixture by
suitably fixturing a quantity of it with the metallic elements, i.e., the
casing and electrodes by positioning casing 13, plug 15, and electrode 14
within fixtures 31. The assembly is then brought to a temperature above
the glass working point, the mixture is allowed to reflow to wet the
metallic surfaces, and finally the assembly is allowed to cool so that a
chemically bonded fusion seal results. This technique allows the use of
electrodes that have been preformed into electrically useful shapes, e.g.,
as helical inductors.
Alternatively, the dried mixture may be reflowed at elevated temperatures
to form desired shapes or "pre-forms" in the configuration of vitreous
solid/cylindrical pellets, toroids, spheres, tubes or wafers with one or
more thru-holes. These pre-forms may be used in conjunction with
high-speed automated machinery to pre-assemble the end-item before it is
submitted to the reflow furnace for fusion sealing. The vitreous pre-forms
must be substantially free of voids to insure uniformity of the
filter/seals that result from their use. They should be sized to provide a
free running fit with respect to the end item casing, and the electrical
conductors. Dimensional tolerances may be relatively loose as long as the
mass of the preform is closely controlled.
EXAMPLE 1
A header subassembly incorporating a filter/seal for use in an
electro-explosive device having a one ohm bridgewire as depicted in FIG. 6
illustrates an implementation of the invention.
The ceramic composition is prepared by mixing the filler, a finely ground
(325 mesh) commercial grade sintered Nickel-Zinc spinel ferrite powder,
(NiO).sub.0.3 (ZnO).sub.0.7 Fe.sub.2 O.sub.3, with binder, a ground (325
mesh) Lead Aluminoborosilicate glass (10% Silica, 10% Boron Oxide, 15%
Aluminum Oxide and 75% Lead Oxide, all by weight), in a polyethylene ball
mill with zirconia or alumina media, polyvinyl alcohol or acetone as the
organic carrier liquid, polyvinyl acetate or polyvinyl butyrol as the
forming agent, and menhaden fish oil as the dispersant. The filler/binder
ratio is 85% by weight. The resulting material is dried, pressed into the
shape of a toroid using a press equipped with a stainless steel die set,
placed on a silica firing plate having a suitable conformal indentation
and vitrified at 590.degree. C. in an oxidizing atmosphere for 45 minutes.
A vitreous toroid shaped pre-form free of organic material is thus
obtained after subsequent cooling and solidification.
Characteristic properties of the fused ceramic material at 25.degree. C.
are given in Table I:
TABLE I
______________________________________
Density 4.6 g/cm.sup.3
Thermal Conductivity 3.5 W/C-m
Specific Heat 0.8 J/g-sec
Thermal Diffusivity 9 .times. 10.sup.-7
m.sup.2 /sec
Thermal Coefficient of Expansion
8.5 ppm/C
Helium Permeability 10.sup.-12
darcys
Curie Temperature 140 C
DC resistivity 10.sup.6 ohm-cm
Dielectric Strength, min.
200 V/mil
RF Properties at 10 MHz
Dielectric Constant 10
Initial Permeability 500
Loss Tangent
magnetic, u"/u' 1
electric, e"/e' 0.1
Unguided Wave Propagation Constant
attenuation constant 5.3 nepers/m
______________________________________
The EED header is manufactured by joining (1) the cylindrical casing
(Iron-Nickel alloy #46 per ASTM F30-85, average linear TCE 7.1-7.8 ppm/C
over 300-350 C, 8.2-8.9 ppm/C over 30-500 C), (2) electrode (DUMET wire
per ASTM F29-78, radial TCE 9.2 ppm/C) in the form of a straight round
wire, and (3) pre-form together on a graphite or Boron Nitride fixture,
and then submitting the loose fitting assembly to a furnace for firing at
600.degree. C. for 10 minutes in an oxidizing atmosphere. The pre-form
melts, reflows within the casing and about the electrode and, with
cooling, solidifies to form the fused filter/seal. The device requires a
further annealing soak at 390.degree. C. for 30 minutes to minimize
microstress formation through the matrix. A slow cool to ambient
temperature completes this portion of the process. Various finishing
operations, such as deburring, grinding, polishing, cleaning and plating
may be required to make the final part useable.
Table II summarizes the performance characteristics of a typical
filter/seal plug constructed as described. The plug has a coaxial geometry
with the dimensions specified.
Table II
______________________________________
Dimensions
Ceramic Plug Length 1.0 cm
Casing Inside Diameter 0.5 cm
Electrode Diameter 0.1 cm
Termination Impedance @ 10 MHz
Real{Z} 1.2 ohm
Imag{Z} 0.2 ohm
Insulation Resistance, min. (1)
5 .times. 10.sup.7
ohms
Dielectric Strength, min. (2)
1000 VDC
Seal Integrity
Helium Leak @ 1 atm. (3)
10.sup.-8
cm.sup.3 /s
Retention, min. 3000 PSI
Feed Point Impedance
Real{Z} 84 ohm
Imag{Z} 81 ohm
RF Attenuation @ 10 MHz (4)
18 dB
______________________________________
EXAMPLE 2
A filter/seal in all respects as in Example #1, but with manganese-zinc
spinel ferrite powder of the form (MnO)0.5(ZnO).sub.0.5 Fe.sub.2 O.sub.3
filler/binder ratio of 60%, and a helical electrode formed as three
complete turns of 0.05 cm diameter wire with a pitch of 0.15 cm, provides
a terminated power loss of approximately 8 dB at 1 Mhz. The efficacy of
the filter/seal declines at higher frequencies, but it offers superior
performance over 0.1 to 1.0 MHz when compared to the filter/seal described
in Example #1.
QUANTITATIVE MECHANICAL AND ELECTRICAL DESIGN CRITERIA
Filter/seals of the invention may be designed to meet a diverse range of
quantifiable performance goals. By selection of the specific binder and
filler, controlling the proportions and particle sizes thereof, adding
property modifying agents and adapting the formulation process, the
following intrinsic material variables may be adjusted to meet the
particular extrinsic requirements of a given application:
(1) linear thermal coefficient of expansion (TCE);
(2) thermal conductivity and diffusivity;
(3) viscous gas flow permeability;
(4) strain point, i.e. the temperature at which the ceramic's viscosity is
10.sup.14.6 poise;
(5) the working point, i.e. the temperature at which the ceramic will
readily flow and wet the metallic surfaces that it comes into contact
with;
(6) Curie point;
(7) DC electrical volume resistivity (DCR);
(8) dielectric strength; and
(9) unguided wave attenuation constant, i.e. the real component of the
complex electromagnetic propagation constant,
##EQU1##
where f is the frequency (Hz), .epsilon.*=.epsilon.'-j.epsilon." is the
complex electric permitivity (farads/meter), and .mu.*=.mu.'-j.mu." is the
complex magnetic permeability (henrys/meter).
1. Thermal Coefficient of Expansion (TCE).
High strength filter/seals require that the TCEs of binder and filler be
closely matched to avoid the development of micro-stresses throughout the
matrix that might lead to microcracking and failure of the seal.
Furthermore, the TCE of the resulting ceramic composition must be properly
related to that of the metals chosen for the end item's electrical
conductors and casing. Preferably, the ceramic matrix material has a
linear expansion coefficient in the range of 3 to 20 ppm/.degree.C. In
general, the seal should be designed so as to insure that the ceramic is
compressively loaded in the vicinity of the metallic members.
Spinel ferrites have TCEs falling within the range of 8 to 10
ppm/.degree.C. The glass binders identified above are specifically
designed to fall within this range. This means that good
thermal-mechanical solutions exist for end items constructed with ASTM
F30-85 Iron-Nickel sealing alloys #46, #48 and #52, which also fall within
this range. Many other commonly available alloys, e.g. #426 stainless
steel (TCE 9.0 ppm/.degree.C.) are also compatible with the TCE range of
the ceramic composition described herein.
Adjustments to the ceramic material formulation may be effected to achieve
TCE matched or compression seals with a variety of metallic casing
materials to include mild carbon, nickel-iron, and stainless steels.
2. Thermal Conductivity and Diffusivity.
The filter/seal achieves its attenuation effect by the thermal dissipation
of RF energy within the plug of ceramic material, but as the temperature
of the filter/seal rises, the effective RF attenuation diminishes,
becoming negligible at and above the Curie point. It is thus desirable
that heat be shed to the environment with maximum efficiency. Since the
thermal contact between the fused ceramic material and the casing is
nearly ideal, it is desirable to formulate the ceramic for maximum thermal
conductivity to facilitate heat transfer from the interior of the plug.
The ceramic materials described have a typical thermal conductivity of 3.5
watts/meter-second.
The dynamic heat transfer properties of the ceramic material are important
for applications where transient RF pulses must be absorbed. Thermal
diffusivities for these materials fall within the range of
5.times.10.sup.-4 to 5.times.10.sup.-2 meters.sup.2 /second.
3. Viscous Gas Flow Permeability.
High quality hermetically sealed electrical connectors typically require
dry air leakage rates that do not exceed 10.sup.-7 cc/s, at 0.5 atmosphere
differential pressure. More stringent requirements, e.g. that helium
leakage rates that do not exceed 10.sup.-8 cc/s, are not uncommon. This
implies that the helium permeability for useful filter/seal ceramic
materials resulting from this invention does not exceed 2.times.10.sup.-11
darcys, and preferably does not exceed 1.times.10.sup.-11 darcys.
The high porosity of the ferrimagnetic and ferroelectric fillers described
is overcome by liquefying the binder glass at elevated temperatures to
wet, coat and infiltrate the filler particles which are thus pulled
together by capillary forces to form a dense, strong glassy matrix.
Thermodynamically, the surface tension between the binder and filler must
be sufficiently low for this mechanism to work. This will be the case
since both are metallic oxides.
4. Strain Point.
The binder's strain point must be well above the end item's highest service
temperature (typically 150.degree. C.) and also above the highest
temperatures required by subsequent end-item assembly processes such as
soldering (typically 200.degree.-400.degree. C.) that might affect the
filter/seal. A lower limit of 300.degree. C. for the annealing point is
achievable for the binders identified. Preferably, the strain point of the
ceramic matrix is in the range of 250.degree. to 700.degree. C.
5. Working Point.
At the opposite extreme, the binder's working point must be well below the
temperature at which the filler melts, commences dissolution into the
glass binder or irreversibly degrades as an electromagnetically lossy
material. For the fillers identified, this requires that the working point
not exceed 1000.degree. C. and should preferably be below 600.degree. C.
Preferably, the working point of the ceramic matrix is in the range of
400.degree. to 1000.degree. C.
6. Curie Point.
The ceramic material's Curie point, primarily a function of the filler
material selected, must exceed the filter/seal's maximum service
temperature by an adequate engineering margin. RF attenuation will
consistently diminish as the Curie temperature is approached and will
vanish altogether at temperatures above the Curie temperature. Preferably,
the Curie temperature of the ceramic matrix is in the range of 130.degree.
to 600.degree. C.
7. DC Resistivity (DCR).
The DCRs of unmodified Borosilicate and Aluminosilicate glasses used in
typical low leakage electrical glass-to-metal seals are in excess of
10.sup.13 ohm-cm at 25.degree. C. and decrease linearly with increasing
temperature. High resistivity is obtained by minimizing alkali content and
employing divalent ions such as lead and barium as modifiers. Cf. Kingery,
et. al., in Introduction to Ceramics (John Wiley & Sons, New York 1976),
pp. 883-4. In contrast, the nominal DCRs of the lossy commercial grade
ferrites cited as fillers range from 10.sup.2 to 10.sup.9 ohm-cm at
25.degree. C. Small percentages of modifiers such as cobalt, manganese and
iron may be employed to increase DCRs for these materials at the expense
of magnetic permeability and decreased Curie point if required. The high
resistivities of the materials described are achieved primarily by
controlling the DCR of the glass binder, and insuring that the more
conductive filler particles are effectively coated by the insulating
glass.
High quality sealed electrical interconnect devices typically require
conductor-to-conductor insulation resistances that exceed 10.sup.8 ohms at
500 VDC, but EEDs that have low resistance pin-to-case bridgewires,
typically 1 to 5 ohms, are satisfactory if the parallel pin-to-case
leakage resistance through the glass seal is as low as 100 ohms. The
compositions described may be adjusted to meet this range of DCR
requirement. Preferably, the DC electrical volume resistivity is in excess
of 100 ohm-cm.
8. Dielectric Strength.
The ceramic materials described have a dielectric strength that
substantially exceeds 150 volts/mil at 25.degree. C. Higher withstand
levels, as may be needed for high voltage feed-thru applications, e.g.,
automotive spark plugs, may be obtained by suitable adjustments in
formulation.
9. Unguided Wave Attenuation Constant.
The filter/seals described will dissipate RF power by multiple mechanisms:
(1) magnetic dissipation in the ceramic due to hysteresis and eddy current
loss, (2) electric absorption in the ceramic due to dielectric relaxation
loss, and (3) ohmic conduction losses in the ceramic and metallic
conductor members. The electromagnetic attenuation constant serves as a
composite figure of merit for the ceramic material's RF dissipation
performance. An extremely wide range of attenuation constants may be
achieved within the described context by adjusting the formulation of the
filler. Fillers based on Nickel-Zinc ferrites may provide attenuations in
the order of 4, 18 and 80 nepers/meter at 0.1, 1 and 10 MHz, respectively,
with appropriate formulation. Preferably, the unguided wave attenuation
constant is greater than 1 neper/meter at 1 MHz, and greater than 5
nepers/meter at 10 MHz and above.
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